1. Field of the Invention
The present invention relates to nonreciprocal circuit elements, and in particular, circulators for use in a microwave band. The present invention also relates to transceiver devices that include circulators and are installed in cellular phones and the like.
2. Description of the Related Art
Conventionally, circulators have a property that a signal is transmitted only in a predetermined direction and is not transmitted in the opposite direction. This property allows the circulator to be used in a transceiver circuit module of mobile communication device such as a cellular phone or the like.
As a circulator of this type, Japanese Unexamined Patent Application Publication No. 9-232818 describes a circulator in which a first center electrode and a second center electrode are arranged on a ferrite, to which a direct-current magnetic field is applied, so that the first center electrode and the second center electrode cross each other while being electrically insulated from each other. Japanese Unexamined Patent Application Publication No. 9-232818 further describes that end portions of the first and second center electrodes form first and second input/output ports, respectively, and that one of the other end portions of the first and second center electrodes forms a third input/output port. This circulator has improved insertion characteristics and isolation characteristics compared with conventional ones.
In coming years, multiband-use and multimode-use of cellular phones may be further advanced to be compatible with multiple communication systems, and these advancements require a cellular phone to have wide-bandwidth such as a factional bandwidth of 30%. However, the circulator has a problem that the isolator characteristic is narrower in view of required wideband use in coming years. That is, in the circulator, a 10-dB isolation bandwidth is 140 MHz with a center frequency of 1.5 GHz, and the fractional bandwidth is about 9% (140/1500).
Further, in the circulator, the center electrode is not connected to ground. Thus, there is a problem that, when being mounted on a circuit board or the like, stray capacitance is formed between the center electrode and the ground of the board and causes the characteristics to shift.